Negative Pressure Vitrification of the Isochorically Confined Liquid in Nanopores

The Dominance of Pretransitional Effects in Liquid Crystal-Based Nanocolloids: Nematogenic 4-methoxybenzylidene-4'-butylaniline with Transverse Permanent Dipole Moment and BaTiO3 Nanoparticles

The report presents static, low-frequency, and dynamic dielectric properties in the isotropic liquid, nematic, and solid phases of MBBA and related nanocolloids with paraelectric BaTiO3 nanoparticles (spherical, d = 50 nm). MBBA (4-methoxybenzylidene-4'-butylaniline) is a liquid crystalline compound with a permanent dipole moment transverse to the long molecular axis. The distortions-sensitive analysis of the dielectric constant revealed its hidden pretransitional anomaly, strongly influenced by the addition of nanoparticles. The evolution of the dielectric constant in the nematic phase shows the split into two regions, with the crossover coinciding with the standard melting temperature. The 'universal' exponential-type behavior of the low-frequency contribution to the real part of the dielectric permittivity is found. The critical-like pretransitional behavior in the solid phase is also evidenced. This is explained by linking the Lipovsky model to the Mossotti catastrophe concept under quasi-negative pressure conditions. The explicit preference for the 'critical-like' evolution of the apparent activation enthalpy is worth stressing for dynamics. Finally, the long-range, 'critical-like' behavior of the dissipation factor (D = tgδ), covering the isotropic liquid and nematic phases, is shown.

Water Promotes Melting of a Metal-Organic Framework

Water is one of the most reactive and abundant molecules on Earth, and it is thus crucial to understand its reactivity with various material families. One of the big unknown questions is how water in liquid and vapor forms impact the fast-emerging class of metal-organic frameworks (MOFs). Here, we discover that high-pressure water vapor drastically modifies the structure and hence the dynamic, thermodynamic, and mechanical properties of MOF glasses. In detail, we find that an archetypical MOF (ZIF-62) is extremely sensitive to heat treatments performed at 460 °C and water vapor pressures up to ∼110 bar. Both the melting and glass transition temperatures decrease remarkably (by >100 °C), and simultaneously, hardness and Young's modulus increase by up to 100% under very mild treatment conditions (<20 bar of hydrothermal pressure). Structural analyses suggest water to partially coordinate to Zn in the form of a hydroxide ion by replacing a bridging imidazolate-based linker. The work provides insight into the role of hot-compressed water in influencing the structure and properties of MOF glasses and opens a new route for systematically changing the thermodynamics and kinetics of MOF liquids and thus altering the thermal and mechanical properties of the resulting MOF glasses.

Studying the Intermolecular Interactions, Structural Dynamics, and Non-Equilibrium Kinetics of Cilnidipine Infiltrated into Alumina and Silica Pores

In the present study, the behavior of the calcium channel blocker cilnidipine (CLN) infiltrated into silica (SiO₂) and anodic aluminum oxide (AAO) porous membranes characterized by a similar pore size (d = 8 nm and d = 10 nm, respectively) as well as the bulk sample has been investigated using differential scanning calorimetry, broadband dielectric spectroscopy (BDS), and Fourier-transform infrared spectroscopy (FTIR) techniques. The obtained data suggested the existence of two sets of CLN molecules in both confined systems (core and interfacial). They also revealed the lack of substantial differences in inter- and intramolecular dynamics of nanospatially restricted samples independently of the applied porous membranes. Moreover, the annealing experiments (isothermal time-dependent measurements) performed on the confined CLN clearly indicated that the whole equilibration process under confinement is governed by structural relaxation. It was also found that the β_anneal parameters obtained from BDS and FTIR data upon equilibration of both confined samples are comparable (within 10%) to each other, while the equilibration constants are significantly different. This finding strongly emphasizes that there is a close connection between the inter- and intramolecular dynamics under nanospatial restriction.

New Insights from Nonequilibrium Kinetics Studies on Highly Polar S-Methoxy-PC Infiltrated into Pores

Herein, the annealing of highly polar (S)-(-)-4-methoxymethyl-1,3-dioxolan-2-one (S-methoxy-PC) within alumina and silica porous membranes characterized by different pore diameters was studied by means of dielectric spectroscopy. We found a significant slowing down of the structural dynamics of confined S-methoxy-PC with annealing time below and, surprisingly, also above the glass transition temperatures of the interfacial layer, T_{g,interfacial}. Furthermore, unexpectedly, a change in the slope of temperature dependencies of the characteristic time scale of this process τ_anneal, at T_{g,interfacial} for all confined samples, was reported. By modeling τ_anneal(T), we noted that the observed enormous variation of τ_anneal results from a decrease of the pore radius due to the vitrification of the interfacial molecules. This indicates that the enhanced dynamics of confined materials upon cooling is mainly controlled by the interfacial molecules.

Interfacial Forces in Free-Standing Layers of Melted Polyethylene, from Critical to Nanoscopic Thicknesses

Molecular dynamics simulations of ultrathin free-standing layers made of melted (373.15–673.15 K) polyethylene chains, which exhibit a lower melting temperature (compared to the bulk value), were carried out to investigate the dominant pressure forces that shape the conformation of chains at the interfacial and bulk liquid regions. We investigated layer thicknesses, tL, from the critical limit of mechanical stability up to lengths of tens of nm and found a normal distribution of bonds dominated by slightly stretched chains across the entire layer, even at large temperatures. In the bulk region, the contribution of bond vibrations to pressure was one order of magnitude larger than the contributions from interchain interactions, which changed from cohesive to noncohesive at larger temperatures just at a transition temperature that was found to be close to the experimentally derived onset temperature for thermal stability. The interchain interactions produced noncohesive interfacial regions at all temperatures in both directions (normal and lateral to the surface layer). Predictions for the value of the surface tension, γ, were consistent with experimental results and were independent of tL. However, the real interfacial thickness—measured from the outermost part of the interface up to the point where γ reached its maximum value—was found to be dependent on tL, located at a distance of 62 Å from the Gibbs dividing surface in the largest layer studied (1568 chains or 313,600 bins); this was ~4 times the length of the interfacial thickness measured in the density profiles.

Non-equilibrium Effects of Polymer Dynamics under Nanometer Confinement: Effects of Architecture and Molar Mass

The non-equilibrium dynamics of linear and star-shaped cis-1,4 polyisoprenes confined within nanoporous alumina is explored as a function of pore size, d, molar mass, and functionality (f = 2, 6, and 64). Two thermal protocols are tested: one resembling a quasi-static process (I) and another involving fast cooling followed by annealing (II). Although both protocols give identical equilibrium times, it is through protocol I that it is easier to extract the equilibrium times, t_eq, by the linear relationships of the characteristic peak frequencies with time and rate, respectively, as log(f_max) = C₁ - k log(t) and log(f_max) = C₂ + λ log(β). Both thermal protocols establish the existence of a critical temperature (at T_c, where k → 0 and λ → 0) below which non-equilibrium effects set-in. The critical temperature depends on the degree of confinement, 2R_g/d, and on molecular architecture. Strikingly, establishing equilibrium dynamics at all temperatures above the bulk, T_g, requires 2R_g/d ∼ 0.02, i.e., pore diameters that are much larger than the chain dimensions. This reflects non-equilibrium configurations of the adsorbed layer that extent away from the pore walls. The equilibrium times depend strongly on temperature, pore size, and functionality. In general, star-shaped polymers require longer times to reach equilibrium because of the higher tendency for adsorption. Both thermal protocols produced an increasing dielectric strength for the segmental and chain modes. The increase was beyond any densification, suggesting enhanced orientation correlations of subchain dipoles.

Experimental and Modeling Comparison of the Dynamics of Capped and Freestanding Poly(2-chlorostyrene) Films

Proximity to a nonrepulsive wall is commonly considered to cause slower dynamics, which should lead to greater relaxation times for capped thin polymer films than for bulk melts. To the contrary, here we demonstrate that Al-capped films of poly(2-chlorostyrene) exhibit enhanced dynamics with respect to the bulk, similar to analogous freestanding films. To quantitatively resolve the impact of interfaces on whole film dynamics, we analyzed the experimental data via the Cooperative Free Volume rate model. We found that the interfacial region adjacent to a cap contains an excess of free volume (relative to the bulk) about half of that proximate to a free surface. Employing a useful analogy between confinement and pressure effects, we estimated that the effect of capping an 18 nm freestanding film would be equivalent to applying a pressure increase of 19 MPa.

The dynamics of freestanding films: predictions for poly(2-chlorostyrene) based on bulk pressure dependence and thoughtful sample averaging

In this paper we model the segmental relaxation in poly(2-chlorostyrene) 18 nm freestanding films, using only data on bulk samples to characterize the system, and predict film relaxation times (τ) as a function of temperature that are in semi-quantitative agreement with film data. The ability to translate bulk characterization into film predictions is a direct result of our previous work connecting the effects of free surfaces in films with those of changing pressure in the bulk. Our approach combines the Locally Correlated Lattice (LCL) equation of state for prediction of free volume values (V_free) at any given density (ρ), which are then used in the Cooperative Free Volume (CFV) rate model to predict τ(T, V_free). A key feature of this work is that we calculate the locally averaged density profile as a function of distance from the surface, ρ_av(z), using the CFV-predicted lengthscale, L_coop(z), over which rearranging molecular segments cooperate. As we have shown in the past, ρ_av(z) is significantly broader than the localized profile, ρ(z), which translates into a relaxation profile, τ(z), exhibiting a breadth that mirrors experimental and simulated results. In addition, we discuss the importance of averaging the log of position dependent relaxation times across a film sample (〈log τ(z)〉), as opposed to averaging the relaxation times, themselves, in order to best approximate a whole sample-averaged value that can be directly compared to experiment.

On the Segmental Dynamics and the Glass Transition Behavior of Poly(2-vinylpyridine) in One- and Two-Dimensional Nanometric Confinement

Geometric nanoconfinement, in one and two dimensions, has a fundamental influence on the segmental dynamics of polymer glass-formers and can be markedly different from that observed in the bulk state. In this work, with the use of dielectric spectroscopy, we have investigated the glass transition behavior of poly(2-vinylpyridine) (P2VP) confined within alumina nanopores and prepared as a thin film supported on a silicon substrate. P2VP is known to exhibit strong, attractive interactions with confining surfaces due to the ability to form hydrogen bonds. Obtained results show no changes in the temperature evolution of the α-relaxation time in nanopores down to 20 nm size and 24 nm thin film. There is also no evidence of an out-of-equilibrium behavior observed for other glass-forming systems confined at the nanoscale. Nevertheless, in both cases, the confinement effect is seen as a substantial broadening of the α-relaxation time distribution. We discussed the results in terms of the importance of the interfacial energy between the polymer and various substrates, the sensitivity of the glass-transition temperature to density fluctuations, and the density scaling concept.

Local structure and molecular dynamics of highly polar propylene carbonate derivative infiltrated within alumina and silica porous templates

Herein, we examined the effect of finite size and wettability on the structural dynamics and the molecular arrangement of the propylene carbonate derivative, (S)-(-)-4-methoxymethyl-1,3-dioxolan-2-one (assigned as s-methoxy-PC), incorporated into alumina and silica porous templates of pore diameters d = 4 nm-10 nm using Raman and broadband dielectric spectroscopy, differential scanning calorimetry, and x-ray diffraction. It was demonstrated that only subtle changes in the molecular organization and short-range order of confined s-methoxy-PC molecules were detected. Yet, a significant deviation of the structural dynamics and depression of the glass transition temperatures, T_g, was found for all confined samples with respect to the bulk material. Interestingly, these changes correlate with neither the finite size effects nor the interfacial energy but seem to vary with wettability, generally. Nevertheless, for s-methoxy-PC infiltrated into native (more hydrophilic) and modified (more hydrophobic) silica templates of the same nanochannel size (d = 4 nm), a change in the dynamics and T_g was negligible despite a significant variation in wettability. These results indicated that although wettability might be a suitable variable to predict alteration of the structural dynamics and depression of the glass transition temperature, other factors, i.e., surface roughness and the density packing, might also have a strong contribution to the observed confinement effects.

Unique Behavior of Poly(propylene glycols) Confined within Alumina Templates Having a Nanostructured Interface

Herein we show that the nanostructured interface obtained via modulation of the pore size has a strong impact on the segmental and chain dynamics of two poly(propylene glycol) (PPG) derivatives with various molecular weights (Mn = 4000 g/mol and Mn = 2000 g/mol). In fact, a significant acceleration of the dynamics was observed for PPG infiltrated into ordinary alumina templates (Dp = 36 nm), while bulklike behavior was found for samples incorporated into membranes of modulated diameter (19 nm < Dp < 28 nm). We demostrated that the modulation-induced roughness reduces surface interactions of polymer chains near the interface with respect to the ones adsorbed to the ordinary nanochannels. Interestingly, this effect is noted despite the enhanced wettability of PPG in the latter system. Consequently, as a result of weaker H-bonding surface interactions, the conformation of segments seems to locally mimic the bulk arrangement, leading to bulklike dynamics, highlighting the crucial impact of the interface on the overall behavior of confined materials.

To Understand Film Dynamics Look to the Bulk

We show that shifts in dynamics of confined systems relative to that of the bulk material originate in the properties of bulk alone, and exhibit the same form of behavior as when different bulk isobars are compared. For bulk material, pressure-dependent structural relaxation times follow τ(T,V)∝exp[f(T)×g(V)]. When two states (isobars) of the material, "1" and "2", are compared at the same temperature this leads to a form τ_{2}∝τ_{1}^{c}, where c=g[V_{2}(T)]/g[V_{1}(T)]. Using equation of state analysis and two models for P-dependent dynamics, we show that c is approximately T independent, and that it can be very simply expressed in terms of either the (free) volume above the close packed state (V_{free}) or the activation energy for cooperative motion. The effect of changing state through a shift in pressure (P_{1} to P_{2}) is thus mechanistically traceable to cooperativity changing with density, through V_{free}. The connection with confined dynamics follows when 1 and 2 are taken as bulk and film at ambient P, differing in density only due to the film surface. The general form for τ(T,V) also illuminates why samples in different states (film vs bulk, high P vs low) trend toward the same relaxation behavior at high T.

Effect of Surface Chemistry on the Glass-Transition Dynamics of Poly(phenyl methyl siloxane) Confined in Alumina Nanopores

Broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) are combined to study the effect of changes in the surface chemistry on the segmental dynamics of glass-forming polymer, poly(methylphenylsiloxane) (PMPS), confined in anodized aluminum oxide (AAO) nanopores. Measurements were carried for native and silanized nanopores of the same pore sizes. Nanopore surfaces are modified with the use of two silanizing agents, chlorotrimethylsilane (ClTMS) and (3-aminopropyl)trimethoxysilane (APTMOS), of much different properties. The results of the dielectric studies have demonstrated that for the studied polymer located in 55 nm pores, changes in the surface chemistry and thermal treatment allows the confinement effect seen in temperature evolution of the segmental relaxation time, τα(T) to be removed. The bulk-like evolution of the segmental relaxation time can also be restored upon long-time annealing. Interestingly, the time scale of such equilibration process was found to be independent of the surface conditions. The calorimetric measurements reveal the presence of two glass-transition events in DSC thermograms of all considered systems, implying that the changes in the interfacial interactions introduced by silanization are not strong enough to inhibit the formation of the interfacial layer. Although DSC traces confirmed the two-glass-transition scenario, there is no clear evidence that vitrification of the interfacial layer affects τα(T) for nanopore-confined polymer.

High-pressure experiments as a novel perspective to study the molecular dynamics of glass-forming materials confined at the nanoscale

Herein, we report the pioneering high-pressure dielectric studies on the dynamics of a model van der Waals glass-forming liquid bisphenol-A diglycidyl ether (DGEBA) infiltrated into anodic aluminum oxide (AAO) templates of the mean pore sizes, d = 150 and d = 18 nm. It was found that although the shape of the structural relaxation process varies with the confinement, it remains constant under varying thermodynamic conditions for a given pore diameter. Consequently, the time-temperature-pressure (TTP) rule satisfied for the majority of bulk substances is also obeyed for the spatially restricted liquid. We have also shown for the first time that there is a decoupling between the core and interfacial mobility at elevated pressure. Moreover, it was noted that the structural dynamics of the former fraction of molecules becomes systematically shorter with respect to the bulk DGEBA during the compression. The enhanced structural dynamics of the core material, as well as the varying pressure coefficients of the glass transition temperature of the interfacial and core molecules, have been discussed in the context of a distinct evolution in their free volume/density packing with respect to the bulk DGEBA, and a change in the interfacial tension, which may lead to the enhanced wettability of the liquid adsorbed onto the pore walls under different thermodynamic conditions. The performed high-pressure measurements offer novel perspectives to explore the combination of two different effects, compression and confinement, which might be a breakthrough in the study of the glass transition phenomenon and the behavior of soft materials confined at the nanoscale.

Testing density scaling in nanopore-confinement for hydrogen-bonded liquid dipropylene glycol

Recently, it has been demonstrated that the glassy dynamics of the molecular liquids and polymers confined at the nanoscale level might satisfy the density scaling law (ρ ^γ /T) with the same value of the scaling exponent, γ, as that determined from the high-pressure studies of the bulk material. In this work, we have tested the validity of this interesting experimental finding for strongly hydrogen-bonded molecular liquid, dipropylene glycol (DPG), which is known to violate the ρ ^γ /T scaling rule in the supercooled liquid bulk state. The results of the independent dielectric relaxation studies carried out on increased pressure and in nanopores, have led to an important finding that when the density change induced by geometrical confinement is not very large, DPG can still obey the density scaling law with the same value of the scaling exponent as that found for the bulk sample. In this way, we confirm that the information obtained from the universal density scaling approach applied to nanoscale confined systems is somehow consistent with the macroscopic ones and that in both cases the same fundamental rules governs the glass-transition dynamics.

Discharge of the Nanopore Confinement Effect on the Glass Transition Dynamics via Viscous Flow

Using dielectric spectroscopy, we demonstrate that confinement-induced changes in the glass transition dynamics, as observed for polymethylphenylsiloxane in alumina nanopores, reveal a pronounced nonequilibrium nature. Our results indicate that glass formers confined to nanopores are able to recover their bulklike mobility. We found that the characteristic time constant of such an equilibration process correlates with an extremely slow viscous flow rate in cylindrical channels of nanometer size. Thus, all the way to equilibrium, confinement effects seen in faster segmental dynamics are released through the viscous flow which eventually helps to eliminate surplus volume gained by nanoconstrained polymers upon cooling.

Connecting 1D and 2D Confined Polymer Dynamics to Its Bulk Behavior via Density Scaling

Under confinement, the properties of polymers can be much different from the bulk. Because of the potential applications in technology and hope to reveal fundamental problems related to the glass-transition, it is important to realize whether the nanoscale and macroscopic behavior of polymer glass-formers are related to each other in any simple way. In this work, we have addressed this issue by studying the segmental dynamics of poly(4-chlorostyrene) (P4ClS) in the bulk and upon geometrical confinement at the nanoscale level, in either one- (thin films on Al substrate) or two- (within alumina nanopores) dimensions. The results demonstrate that the segmental relaxation time, irrespective of the confinement size or its dimensionality, can be scaled onto a single curve when plotted versus ρ^γ/T with the same single scaling exponent, γ = 3.1, obtained via measurements at high pressures in bulk. The implication is that the macro- and nanoscale confined polymer dynamics are intrinsically connected and governed by the same underlying rules.

Efficient metal-free strategies for polymerization of a sterically hindered ionic monomer through the application of hard confinement and high pressure

In this paper, we have studied the effect of both hard confinement (nanoporous membranes treated as nanoreactors) and high pressure (compression of system) on the progress of free-radical (FRP) and reversible addition-fragmentation chain transfer (RAFT) polymerizations of selected hardly polymerizable, sterically hindered imidazolium-based ionic monomer 1-octyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide ([OVIM][NTf₂]). These two innovative approaches, affecting (in a different way) the free volume of the polymerizing system, allows the reduction of the number of toxic substrates/catalysts, satisfying the requirement of green chemistry. It was found that at both conditions (high compression and confinement) the polymerizability of monomer, as well as the control over the reaction and the properties of the produced polyelectrolytes, have increased significantly. However, it should be added that there were noticeable differences between FRP carried out under confinement and at high pressures. Interestingly, by appropriate variation in thermodynamic conditions, it was possible to synthesize polymers of moderate molecular weight (M_n ∼ 58 kg mol⁻¹) and relatively low dispersity (Đ ∼ 1.7); while for the reaction performed within AAO pores of varying diameter (d = 35 nm and d = 150 nm), macromolecules of higher M_n but slightly broader dispersity indices (Đ ∼ 2.2–2.7) were recovered. On the other hand, RAFT polymerization carried out under confinement and at elevated pressures yielded polymers with well-defined properties. Noteworthy is also the fact that nanopolymerization leads to polymers of comparable M_n to those obtained at high-pressure studies but at significantly shorter reaction time (t ∼ 2 hours). We believe that the presented data clearly demonstrated that both examined approaches (the compression and application of alumina templates, treated as nanoreactors) could be successfully used as additional driving forces to polymerize sterically hindered monomers and produce well-defined polymers in relatively short times. At the same time, it should be mentioned that both proposed polymerization methods enabled us to omit the addition of metal-based initiators/catalysts, which seem to be a crucial step towards further development of the alternative green synthesis of polyelectrolytes in the future.

The effect of hard confinement and high pressure on the progress of free-radical and reversible addition-fragmentation chain transfer polymerizations of sterically hindered 1-octyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide ([OVIM][NTf2]) has been investigated.

Experimental Test of the Cooperative Free Volume Rate Model under 1D Confinement: The Interplay of Free Volume, Temperature, and Polymer Film Thickness in Driving Segmental Mobility

We show that the Cooperative Free Volume (CFV) rate model, successful at modeling pressure-dependent dynamics, can be employed to describe the temperature and thickness dependence of the segmental time of polymers confined in thin films (1D confinement). The CFV model is based on an activation free energy that increases with the number of cooperating segments, which is determined by the system's free volume. Here, we apply the CFV model to new experimental results on the segmental relaxation of 1D confined poly(4-chlorostyrene), P4ClS, and find remarkable agreement over the whole temperature and thickness ranges investigated. This work further validates the robustness of the CFV model, which relates the effects of confinement on dynamics to pressure changes in the bulk, and supports the idea that confinement effects originate from local perturbations in density.

Glassy dynamics of two poly(ethylene glycol) derivatives in the bulk and in nanometric confinement as reflected in its inter- and intra-molecular interactions

The inter- and intra-molecular interactions as they evolve in the course of glassy solidification are studied by broadband dielectric-and Fourier-transform infrared-spectroscopy for oligomeric derivatives of poly(ethylene glycol) derivatives, namely, poly(ethylene glycol) phenyl ether acrylate and poly(ethylene glycol) dibenzoate in the bulk and under confinement in nanoporous silica having mean pore diameters 4, 6, and 8 nm, with native and silanized inner surfaces. Analyzing the spectral positions and the oscillator strengths of specific IR absorption bands and their temperature dependencies enables one to trace the changes in the intra-molecular potentials and to compare it with the dielectrically determined primarily inter-molecular dynamics. Special emphasis is given to the calorimetric glass transition temperature T_g and T_αβ ≈ 1.25T_g, where characteristic changes in conformation appear, and the secondary β-relaxation merges with the dynamic glass transition (α-relaxation). Furthermore, the impact of main chain conformations, inter- and intra-molecular hydrogen bonding, and nanometric confinement on the dynamic glass transition is unraveled.

Glassy dynamics of polymethylphenylsiloxane in one- and two-dimensional nanometric confinement-A comparison

Glassy dynamics of polymethylphenylsiloxane (PMPS) is studied by broadband dielectric spectroscopy in one-dimensional (1D) and two-dimensional (2D) nanometric confinement; the former is realized in thin polymer layers having thicknesses down to 5 nm, and the latter in unidirectional (thickness 50 μm) nanopores with diameters varying between 4 and 8 nm. Based on the dielectric measurements carried out in a broad spectral range at widely varying temperatures, glassy dynamics is analyzed in detail in 1D and in 2D confinements with the following results: (i) the segmental dynamics (dynamic glass transition) of PMPS in 1D confinement down to thicknesses of 5 nm is identical to the bulk in the mean relaxation rate and the width of the relaxation time distribution function; (ii) additionally a well separated surface induced relaxation is observed, being assigned to adsorption and desorption processes of polymer segments with the solid interface; (iii) in 2D confinement with native inner pore walls, the segmental dynamics shows a confinement effect, i.e., the smaller the pores are, the faster the segmental dynamics; on silanization, this dependence on the pore diameter vanishes, but the mean relaxation rate is still faster than in 1D confinement; (iv) in a 2D confinement, a pronounced surface induced relaxation process is found, the strength of which increases with the decreasing pore diameter; it can be fully removed by silanization of the inner pore walls; (v) the surface induced relaxation depends on its spectral position only negligibly on the pore diameter; (vi) comparing 1D and 2D confinements, the segmental dynamics in the latter is by about two orders of magnitude faster. All these findings can be comprehended by considering the density of the polymer; in 1D it is assumed to be the same as in the bulk, hence the dynamic glass transition is not altered; in 2D it is reduced due to a frustration of packaging resulting in a higher free volume, as proven by ortho-positronium annihilation lifetime spectroscopy.

Comparison of high pressure and nanoscale confinement effects on crystallization of the molecular glass-forming liquid, dimethyl phthalate

High pressure and nanoscopic confinement are two different strategies commonly employed to modify the physicochemical properties of various materials. Both strategies act mostly by changing the molecular packing. In this work, we performed a comparative study on the effect of compression and confined geometry on crystallization of a molecular liquid. Dielectric spectroscopy was employed to investigate the crystallization of the van der Waals liquid, dimethyl phthalate, in nanoporous alumina of different pore sizes as well as on increased pressure (up to 200 MPa). The analysis of the crystallization kinetics under varying thermodynamic conditions revealed that both strategies affect the crystallization behavior of the sample in very distinct ways. Compression shifts the maximum crystallization rate towards a higher temperature and broadens it. As a result, it is more challenging to avoid crystallization upon cooling the liquid at high pressure. In contrast, when the same material is incorporated into nanopores, crystallization significantly slows down and the maximum rate shifts towards a lower temperature with decreasing pore size. Finally, we show that crystallization in nanoporous alumina is accompanied by pre-crystallization effects upon which a shift of the α-relaxation peak is observed. An equilibration process prior to the initiation of crystallization was detected for the confined material both above and below the glass transition temperature of the interfacial layer, while not in the bulk.

Studies on the hard confinement effect on the RAFT polymerization of a monomeric ionic liquid. Unexpected triggering of RAFT polymerization at 30 °C

For the first time, the RAFT polymerization of a monomeric ionic liquid under hard confinement was successfully carried out.

Nanostructures and Dynamics of Isochorically Confined Amorphous Drug Mediated by Cooling Rate, Interfacial, and Intermolecular Interactions

The production and stabilization of amorphous drugs by the nanoconfinement effect has recently become a research hotspot in pharmaceutical sciences. Herein, two guest/host systems, indomethacin (IMC) and griseofulvin (GSF) confined in anodic aluminum oxide (AAO) templates with different pore diameters (25-250 nm) are investigated by differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). The crystallization of the confined drugs is suppressed, and their glass transition temperatures show an evident pore-size dependency. Moreover, a combination of dielectric and calorimetric results demonstrates that the significant change in the temperature dependence of the structural relaxation time during the cooling process is attributed to the vitrification of the interfacial molecules and the local density heterogeneity under isochoric confinement. Interestingly, compared with the case of IMC/AAO, which can be described by a typical two-layer model, GSF/AAO presents an rare scenario of three glass transition temperatures under fast cooling (40-10 K/min), indicating that there exists a thermodynamic nonequilibrium interlayer between the bulk-like core and interfacial layer. In contrast, the slow cooling process (0.5 K/min) would lead confined GSF into the stable core-shell nanostructure. Using surface modification, the interfacial effect is confirmed to be an important reason for the different phenomena between these two guest/host systems, and intermolecular hydrogen bonding is also suggested to be emphasized considering the long-range effect of interfacial interactions. Our results not only provide insight into the glass transition behavior of geometrically confined supercooled liquids, but also offer a means of adjusting and stabilizing the nanostructure of amorphous drugs under two-dimensional confinement.

Predicting Nanoscale Dynamics of a Glass-Forming Liquid from Its Macroscopic Bulk Behavior and Vice Versa

The properties of a molecular liquid confined at the nanometer length scale can be very distinct from the bulk. For that reason, the macro- and the nanoscopic behaviors of glass-forming liquids are regarded as two nonconnected realms, governed by their own rules. Here, we show that the glassy dynamics in molecular liquids confined to nanometer pores might obey the density scaling relation, ρ^γ/T, just like in bulk fluids. Even more surprisingly, the same value of the scaling exponent γ superposes the α-relaxation time measured at different state points in nanoscale confinement and upon increased pressure. We report this remarkable finding for van der Waals liquids tetramethyl-tetraphenyl-trisiloxane (DC704) and polyphenyl ether (5PPE), considered as simple, single-parameter liquids. Demonstrating that the density scaling idea can be fulfilled in both environments opens an exciting possibility to predict the dynamic features of the nanoconfined system close to its glass-transition temperature from the high-pressure studies of the bulk liquid. Likewise, we can describe the viscous liquid dynamics at any given combination of temperature and pressure based on analysis of its behavior in nanopores.

Polymerization of Monomeric Ionic Liquid Confined within Uniaxial Alumina Pores as a New Way of Obtaining Materials with Enhanced Conductivity

Broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) have been employed to probe dynamics and charge transport of 1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide ([bvim][NTf₂]) confined in native uniaxial AAO pores as well as to study kinetics of radical polymerization of the examined compound as a function of the degree of confinement. Subsequently, the electronic conductivity of the produced polymers was investigated. As observed, polymerization carried out at T = 363 K proceeds faster under confinement with some saturation effect observed for the sample in pores of smaller diameter. Obtained results were discussed in the context of the very recent reports showing that the free volume of the confined material is higher with respect to the bulk one. It was also noted that conductivity of poly[bvim][NTf₂] is significantly higher with respect to the macromolecules obtained upon bulk polymerization. Moreover, charge transport of the confined macromolecules is even higher when compared to the bulk monomeric ionic liquid at some thermodynamic conditions. Additionally, the molecular weight, M_w, of the confined-synthesized polymers is significantly higher with respect to the bulk-synthesized material. Interestingly, both parameters, (i) the enhancement of σ_dc and (ii) the increase in M_w, can be tuned and controlled by the application of the appropriate confinement. Consequently, those results are quite promising in the context of development of the fabrication of polymerized ionic liquids (PILs) nanomaterials with unique properties and morphologies, which can be further easily applied in the field of nanotechnology.

The peculiar behavior of the molecular dynamics of a glass-forming liquid confined in native porous materials - the role of negative pressure

In this paper, we combine Broadband Dielectric Spectroscopy (BDS) at ambient and high pressure, and positron annihilation lifetime spectroscopy (PALS) data of 2-ethylhexanol in the bulk state and when infiltrated in native silica nanopores to elucidate the relative role of surface effects on the Debye and structural relaxation processes under 2D spatial constraints. We show that the two processes have different sensitivities to (i) the changes in density as quantified by the EV/Hp ratio and (ii) the degree of confinement. Significant enhancement of the dynamics of the confined molecules at low temperatures is related to the vitrification of the interfacial molecules (Tg,int) affecting the packing density of the core molecules. This is corroborated by the PALS measurements, which demonstrated that the effective volume for the confined samples is slightly higher and seems to be temperature invariant below Tg,int. Consequently, negative pressure systematically develops with lowering temperature reaching values of -100 and -110 MPa (depending on the pore size) at the glass transition temperature. This result offers a better understanding of the counterbalance between surface and finite size effects as well as the role of negative pressure in controlling the dynamics and the glass transition of liquids under 2D spatial restrictions.